How Bocoup created their style guide for consistency across all of their products and how they involved the various teams in decisions making along the way.
AdRoll are starting to document the journey endured in creating their style guide. There's some great nuggets in this Q&A
A great and lengthy article on the ideas behind Pattern Lab and how you can use it.
How Pinterest looked to unify and simplify the experience of using the service with a new design system and how they approached it.